ALSA: firewire-lib: unify packet handler for IR context

Usage of 16 bytes IR context header allows to handle context payload by
the same code for with-CIP and without-CIP packets.

This commit unifies both handlers of with-CIP and without-CIP packets.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>